Computed tomography is an imaging method used primarily for medical diagnosis and for the examination of material. In computed tomography, to record spatially three-dimensional image data, a radiation source, for example, an X-ray source and an X-ray detector interacting with said source rotate about an object to be examined. During the rotational movement, scan data is recorded within an angular sector. The projection scan data is a projection or a plurality of projections, which contain information about the attenuation of the radiation by the examination object from different projection angles. A two-dimensional image slice or a three-dimensional volume image of the examination object can be calculated from these projections. The projection scan data is also referred to as raw data or the projection scan data can be preprocessed so that, for example, detector-induced differences in attenuation intensity are reduced. Image data can then be reconstructed from this projection scan data, for example by way of so-called filtered back projection or by way of an iterative reconstruction method.
Numerous methods for scanning an examination object with a computed tomography system are known. For example, orbital scans, sequential orbital scans, with advance or spiral scans are employed. Other types of scan that are not based on orbital movements are also possible, such as scans with linear segments for example. Absorption data of the examination object is recorded from different imaging angles with the aid of at least one X-ray source and at least one X-ray detector lying opposite and this absorption data or projections collected in this way are computed into image slices through the examination object by way of corresponding reconstruction methods.
Computed tomography can use counting directly converting X-ray detectors. In directly converting X-ray detectors, the X-rays or the photons can be converted into electrical pulses by a suitable converter material. The converter material used can, for example, be CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs or other materials. The electric pulses are evaluated by an evaluation electronics system, for example an integrated circuit (Application Specific Integrated Circuit, ASIC).
In counting X-ray detectors, incident X-rays are measured by counting the electric pulses which are triggered by the absorption of X-ray photons in the converter material. The level of the electric pulse is generally proportional to the energy of the absorbed X-ray photon. This enables spectral information to be extracted by comparing the level of the electric pulse with a threshold value. When energy-resolving or energy-selective counting X-ray detectors, for example directly converting X-ray detectors, are used, the measured datasets can be used as the basis for splitting material into two or three materials.
Publication DE 10 2015 205 301 A1 discloses a method for operating a counting digital X-ray image detector, wherein each pixel element and/or each pixel cluster is embodied as switchable between a first counting mode and a second counting mode that is different from the first.
Publication DE 10 2015 206 630 A1 discloses a multi-spectral imaging method, preferably a CT imaging method, wherein spectrally resolved projection scan data is acquired from a region to be imaged of an examination object and said data is assigned to a plurality of predetermined different partial spectra.
Publication DE 10 2007 046 514 A1 discloses a method and an image evaluation unit for recognizing and marking contrast agents in blood vessels of the lung with the aid of a CT examination using at least two different X-ray energy spectra. The method includes the following method steps:
scanning a patient at least in the region of the lung with two different X-ray energy spectra with the patient having contrast agents in the blood stream,
reconstructing at least one two- or three-dimensional tomographic display for each X-ray energy spectrum, which reproduces the local spectrum-specific absorption properties of the scanned region, wherein
a surrounding region is defined for a plurality of voxels for which an average ratio is calculated with the aid of the local absorption values of the at least two X-ray energy spectra, which specifies a measure for the ratio of the proportion of contrast agent to the proportion of soft tissue in the region surrounding the respective voxel under consideration, and,
in the case of the ratio dropping below a prespecified threshold value, this voxel is considered to have reduced circulation and is marked in a tomographic display.
It is generally necessary to decide the fixed recording parameters, for example tube voltage, to be used for recording a region of interest of an object to be examined with a computed tomography system before starting the recording. Dynamic changes or adjustments to recording parameters are not made during the recording of a region of interest of an object to be examined with a computed tomography system. This enables larger regions to be used for recording a region of interest of an object to be examined with a computed tomography system with suboptimal recording parameters.